As feature size decreases, density increases, and the size of semiconductor wafers or substrates increase, Chemical Mechanical Planarization (CMP) process requirements become more stringent. Substrate to substrate process uniformity as well as intra-substrate planarization uniformity are important issues from the standpoint of producing semiconductor products at a low cost. As the size of dies increases a flaw in one small area increasing results in rejection of a relatively large circuit so that even small flaws have relatively large economic consequences in the semiconductor industry.
Many factors are known in the art to contribute to uniformity problems. These include distribution of a slurry between a surface of the substrate and polishing surface during the polishing operation when there is relative motion between a polishing head on which the substrate is held and the polishing surface during the polishing operation. Slurry is a, usually, chemically active liquid having an abrasive material suspended therein that is used to enhance the rate at which material is removed from the substrate surface.
One problem with slurry distribution in a conventional CMP apparatus a non-uniform distribution of slurry on a polishing surface. FIG. 1 is a top plan view of a platen and a slurry dispenser in a conventional CMP apparatus illustrating a non-uniform distribution of slurry on a polishing surface. Referring to FIG. 1, it is seen that distribution of a slurry 10 across a polishing surface 12 is primarily dependent on the location and orientation of an opening or nozzle 14 of a tube 16 dispensing slurry onto the polishing surface, and on the movement or rotation of a platen (not shown) on which the polishing surface 10 is supported. The speed of movement of the platen is generally determined based on a desired polishing rate, that is a rate at which material is removed from a substrate (not shown) being polished. Thus, traditional approaches to providing an adequate and uniform distribution of slurry between a substrate and a polishing head 18 on which the substrate is held have focused on the location and orientation of the nozzle 14 relative to the polishing head.
As illustrated in FIG. 1, if the nozzle 14 dispenses the slurry too far in radially from an edge 20 of the polishing surface 10 or platen, a portion of the polishing surface beneath the polishing head 18 that is nearest to a center 22 of the polishing surface receives the greatest amount of slurry. As a result, the surface of the substrate near an outer circumferential edge of the polishing head 18 has a higher removal rate than the surface near the center. This pattern is further exacerbated by deformation of the polishing surface 10 by the polishing head 18, which causes the slurry near the edge of the polishing head to be deflected or redirected towards away from the polishing head as shown in FIG. 1.
One prior art approach attempting to provide a more uniform distribution of slurry is described in U.S. Pat. No. 5,709,573, to Guthrie et al. (GUTHRIE). GUTHRIE discloses a radially positioned flexible member in contact with the polishing surface to sweep the slurry across the polishing surface. While an improvement over conventional slurry dispensers, this approach is not wholly satisfactory for a number of reasons.
One problem with the approach taught in GUTHRIE is that the constant contact between the flexible member and the polishing surface during polishing operations causes rapid wear of the flexible member. This in turn leads to the need to frequently replace the flexible member. In addition to the cost of replacement parts, this results in excessive down time or loss of availability or the apparatus for processing due to the time needed to replace the flexible member and the time need to re-characterize the polishing process or apparatus. Moreover, prior to replacement, as the flexible member wears the amount and distribution of slurry across the polishing surface can vary introducing a new source of non-uniformity. This is particularly a problem with polishing surfaces comprising a pattern of features, such as indentations in a porous polishing surface or concentric grooves, for aiding in slurry distribution. These features cause the flexible member to wear unevenly across the surface in contact with the polishing surface, resulting in a nonuniform distribution of slurry across the polishing surface.
Another problem with conventional CMP apparatuses and methods, related to the problem with non-uniform distribution described above, is the inefficient use and wastage of slurry. Because the slurry is dispensed onto the polishing surface ahead of the polishing head, an excess of slurry must typically be dispensed to ensure that when it flows across the polishing surface it will cover the entire area between the substrate and the surface. Because of strict requirements concerning the purity of the slurry and in particular the size of the abrasive particles suspended therein, slurry tends to be expensive. Moreover, because materials used in fabricating semiconductors are often hazardous to people and to the environment, used slurry, which can contain significant amounts of material removed from the substrates, must be disposed of as hazardous waste. Thus, a significant factor in the cost of operating conventional CMP apparatuses is the cost of supplying and disposing of the slurry.
Yet another problem with conventional CMP apparatuses and methods is the buildup of solid polishing byproducts on the polishing surface that can damage or destroy a substrate being polished. These byproducts include material removed from the surface of the substrate and agglomerations of abrasives from old or dried out slurry. This particularly a problem for CMP apparatuses including polishing surfaces with numerous small, shallow grooves for the distribution of slurry, or porous polishing pads or coverings.
Accordingly, there is a need for an apparatus and method that provides a controlled or uniform distribution of slurry across the polishing surface to provide improved planarization uniformity. There is a further need for an apparatus and method capable of restricting slurry dispensed on the polishing surface to the portion of the polishing surface over which the polishing head passes during the polishing operation, thereby reducing waste of slurry. There is a yet further need for an apparatus and method capable of removing used slurry and polishing byproducts from the polishing surface thereby eliminating buildup of solid polishing byproducts that can damage the substrate.